Technical analysis of paintings is intended to detect retouching, pentimenti, underdrawings and to identify original and added material (e.g. pigments, binding media, coatings, retouching). This information is essential for dating and authentication of the artwork and contributes significantly to our understanding of art objects. In addition, it facilitates the evaluation of the physical condition (deterioration, interventions) and directs conservation decisions.
Traditionally, the analysis of paintings has been restricted to invasive investigations and is carried out ex situ. This approach has the drawback of being harmful to the painting—as it requires samples to be taken—and provides only spot specific information that is not necessarily representative of the object in its context. Consequently the development of non-invasive techniques that can be used in situ and provide global information will enjoy a great impact.
Over the last two decades there have been substantial advances made in the application of modern scientific techniques to the chemical and structural analysis of works of art. However there is still room for improvement as the analysis of artworks is generally a very complex and demanding problem. A very important issue in this respect is the development and use of non-destructive analytical techniques, which can be applied in situ. In response to the demand for non-destructive analytical devices in the art conservation field, a variety of imaging and spectroscopic techniques have been developed and used for the in situ examination of artwork materials.
Film photography has been extensively used to capture artwork images in the visible, near ultraviolet (NUV) and near infrared (NIR) parts of the spectrum. A variety of different imaging devices have also been used, ranging from film to analog (video) and digital cameras. Moreover, different cameras with spectral sensitivities restricted either to the visible or to the near infrared (NIR) part of the spectrum have been used in order to obtain diagnostic information for the artwork under analysis. In particular, imaging in the near infrared enables the visualization of underdrawings, relying on the phenomenon that in general the overlying pigments become transparent in this wavelength range. Broadband fluorescence photography provides information for the condition of coatings (such as varnishes) and enables the localization of previous restoration interventions. A variety of light dispersion spectroscopic techniques have also been used in situ and ex situ for the identification of painting materials. Materials with the same color appearance, determined by the similar diffuse reflectance spectra in the visible, may have different spectral patterns outside the visible part of the spectrum. Compositional alterations associated with the material deterioration can be recorded by measuring absorption, fluorescence, or (elastic, non-elastic) scattering signals, providing quantitative diagnostic information. Fluorescence and diffuse reflectance spectra are in general broad in the visible and the NIR part of the spectrum due to the complicated nature of the artwork material and due to light-material interaction mechanisms that are involved in these phenomena. Owing to this fact, these spectroscopic techniques are suitable for detecting in situ chemical and structural alterations and for the differentiation of pigments with the same color appearance but different chemical composition. Raman and FTIR spectroscopy provide improved, molecular-specific diagnostic information, since the acquired spectra contain fingerprint information for a specific point area of the artwork material under analysis. Until now the use of these techniques have been restricted to the experimental-ex situ analysis of material samples, mainly due to the required complicated instrumentation. Laser Induced Breakdown Spectroscopy LIBS) is a novel promising technique for in situ analysis, it requires however laser ablation of a spot area and the subsequent spectroscopic analysis of the created plume. For this reason this technique is considered as minimally invasive and in several cases (e.g. fragile and thin material) it is not applicable. Apart from the above mentioned, the common problem that restricts seriously the applicability of conventional spectroscopic methods to the in situ analysis of artwork is that they provide point information, which is inadequate in cases where complicated materials, characterized by a high spatial variability of their contextual features are examined. Moreover the point area under analysis has to be determined by the user, which in several cases is not capable of detecting and focussing his attention in artwork areas that are subjected to alterations. This results in a reduction of the accuracy of these methods due to probing errors.
Summarizing the above mentioned, conventional spectrometers provide a large amount of spectroscopic (analytical) information about one localized site of the object, whereas conventional broadband imaging provides a modest amount of spectral information (resolution), but for a significant area of the object.
In the field of art conservation, there are applications reported where cameras sensitive in the visible and in the NIR are filtered with optical filters, thus enabling the selection of the imaging center wavelength with the aid of a filter tuning mechanism. In the visible part of the spectrum these cameras are used for accurate color reproduction, while in the NIR part of the spectrum filter tuning enables the determination of the appropriate imaging band, at which the maximum imaging information for the underlying features is obtained. Based on the above mentioned it reasonable to suggest that the combination of the advantages of both imaging and spectroscopy will constitute a significant step forward in non destructive analysis and documentation of art-objects and monuments. Although Raman, FTIR and LIBS spectroscopies provide improved analytical information it is very difficult, or in the LIBS case impossible, to operate in imaging mode. In contrast, there is not any fundamental or technological restriction for the development of imaging systems capable of capturing diffuse reflectance and/or fluorescence spectroscopic information for the entire surface under examination. Of course, as mentioned above, these techniques suffer from the main drawback that the captured spectra contain pure information or painting material identification.